In the various industries, particularly in the petrochemical process industry, systems are used wherein process equipment likely to cause hazards upon failure is provided with valves and other mechanical means designed to bring the process quickly into a safe state if predetermined process parameter limits are exceeded. For example, such a system usually includes valves with single-action actuators, the opening or closing of which releases a built-up overpressure, diverts a hazardous process stream into a holding tank, or the like. Hereinafter, such valves are referred to as shutdown valves (being emergency valves having a closing or an opening function). These valves normally maintain one position, like other corresponding mechanical safety devices, and are consequently at risk for getting stuck if situations forcing a shutdown are not, as is hopefully the case, very frequent. The general safety of shutdown valves is not considered satisfactory when prior art equipment and methods are used. The greatest disadvantage in present systems is, that an existing failure for example, a mechanical component getting stuck--is not necessarily observed when the system is in a standby state, and in an emergency situation the system may be impaired or inoperative.
To verify performance, it is common to test e.g. shutdown valves in a manner simulating a real emergency situation. This practice may in fact cause huge risks, as the workability of the shutdown valve system is temporarily blocked, and if the device is not properly activated, the simulated emergency may become an actual emergency.
One method of testing a shutdown valve is to mechanically limit its travel, thus preventing it from having any significant effect on the process, and check the mobility of the valve within certain limits. This procedure requires the use of, for example, a physical key, and activation of the emergency system is prevented during the testing procedure, at least as regards the unit being tested.
The test is carried out at predetermined intervals, for example twice a year, but the test only proves that the devices are workable at the moment of testing. A fault may develop shortly after the test and remain until the next test. This manner of testing is not a reliable way of verifying operability of the system.
Valve diagnostics using present sensor and digital technology is a fast developing field. For example, in Finnish patent application 96 2406 and European patent application 95 306546, methods are disclosed for surveillance of the condition of control valves using sensors in the actuator and control means of the valve and the analysis of signals from said sensors in a microprocessor to analyze the workability of the valve unit as a whole.
As described above, the surveillance of the condition of a valve or other mechanical device which is part of a safety system involves certain requirements.